One common theme is mystery. Since both Gaster and Sans have mysterious backstories in the original game, fanfictions often explore and expand on these mysteries, especially in relation to their relationship.
At first, Sans would probably be a bit shocked, but then he'd turn on his laid - back, sarcastic attitude. He might make jokes at Gaster's expense, which could either annoy Gaster or make him respect Sans more. If Gaster tries to harm him, Sans could use his shortcuts to dodge attacks and try to find a way to reverse the situation. He'd also likely be thinking about how this affects his friends in the Underground, especially Papyrus, and try to find a way back to them.
One possible reason could be for some sort of experiment. Gaster is often depicted as a mysterious scientist in the Undertale lore, and Sans has unique abilities. Maybe Gaster wants to study Sans' power over time and space. Another reason could be related to secrets. Sans might know something about Gaster's past or the true nature of the Underground that Gaster wants to keep hidden, so he kidnaps Sans to prevent him from revealing it.
A sense of otherworldliness is common too. Since Gaster Sans is from a rather strange and unique universe, the fanfictions often play with the idea of the reader being in a very different, almost alien world. This can lead to themes of adjustment, as the reader tries to fit into this new world while being in a relationship with Gaster Sans.
Well, it could be about Gaster suddenly reappearing in the lives of Sans and Papyrus. Maybe he has some important knowledge or a mission for them. For example, he might need their help to fix something in the Underground that's going wrong.
Obviously, Sans and Papyrus are the two main characters who we already know well from the original Undertale. They have distinct personalities that will play a big role in how they interact with Gaster. Gaster, on the other hand, is the enigma. His presence will likely drive the story as Sans and Papyrus try to figure out who he is now and what he wants, so he's also a main character in this fanfiction.
First, focus on the build - up. Create suspense before the transformation. Hint at what's going to happen with strange events or dialogue. Second, describe the transformation vividly. Use strong adjectives to show how Sans is changing. Third, give the transformation a purpose. It can't just be for show. Maybe he's transforming to save his friends or to face a powerful enemy.
One popular 'gaster x sans fanfiction' is 'The Lost Experiments'. It delves into the mysterious relationship between Gaster and Sans, exploring their shared past in the Underground. The story is full of intrigue as it uncovers secrets that were long buried.
One key element could be the visual transformation. The Gaster Blaster Sans might change in appearance, with his blasters taking on new forms or emitting different colors. Another aspect could be the power boost. Maybe his attacks become more powerful or have new effects. Also, the emotional state during the transformation can play a role. Is he enraged, desperate, or something else that triggers this change?
In a 'chara vs gaster fanfic', if the victory is determined by who has the stronger will, it could be a long - drawn - out battle. Chara has a strong will born out of their experiences, but Gaster's will might be equally strong considering his enigmatic nature and whatever his goals are in the story. It's really up to the creativity of the writer to decide who emerges victorious.